Very often, it is necessary to test for any one of a number of reasons both in the field and prior to being shipped the single phase and multiphase voltage apparatus, i.e., transformer and/or its surge arrester. For example, after a transformer is subjected to high voltage transients, which could damage or destroy it, it may be necessary to conduct tests in order to determine whether a part should be replaced. During manufacture there could be faulty connections or the like. Therefore, it may be necessary to test the transformer after manufacture and before shipment. It may also be desirable, and often necessary to conduct routine tests on the transformer in order to determine that it is in good working order.
Oil filled transformers and metal oxide varistor arresters are known. Generally, it is necessary to provide an arrester or a surge protector which protects the transformer against high voltage transients. For this reason, it is common practice to connect an arrester which will conduct transients from a power line to ground ahead of or at the transformer when high voltage occurs. The surge arrester may be mounted within the transformer tank.
High voltage surges actuate the arrester so that damaging electrical potentials are shunted to ground via the arrester before the transformer can be destroyed. Since the internally mounted arrester provides a path for shunting high voltage to ground, it also prevents a valid dielectric test of the transformer insulation system. Thus, it is not possible to test the transformer without disconnecting the internal arrester.
Therefore, the common practice is to disconnect the arrester, dielectrically test the transformer, and then reconnect the arrester or surge protector. In the case of an oil filled transformer which has an arrester mounted therein, it is both awkward and costly to test the transformer and/or arrester. The transformer tank must be opened to so disconnect and reconnect the surge protector. This therefore substantially eliminates field and/or installation evaluation of the transformer.
Still another condition which leads to cost problems and design restrictions is the need heretofore wherein an arrester failure should result in an open circuit fault. For example, most arresters are designed to melt open an isolating fuse link or to fracture and result in an open circuit condition when a transient persists for a period of time. Thereafter, it is necessary to disassemble the transformer and clean and remove all of the arrester parts from the transformer housing. This is especially difficult when the transformer housing is filled with oil. Further, when an under oil arrester mounted in an oil filled transformer fails there is no readily visible means to indicate the arrester failure.